Isocyanate terminated urethane prepolymers are widely used, for instance in liquid moisture curable adhesives and sealants, and as crosslinkers in two component adhesives and sealants. They are also used in the manufacture of hot melt moisture curable adhesives (HMMC). The typical procedure for making isocyanate prepolymers with aromatic polyisocyanates, such as toluene diisocyanate or methylene diphenyl diisocyanate (MDI), and a polyol is usually accomplished at relatively low process temperatures of 70.degree. to 100.degree. C. However, prepolymers made with the conventional 4,4'-MDI isomer, i.e. 4,4'-diphenylmethane diisocyanate, and hydroxy terminated polybutadiene at these process temperatures are hazy, pearlescent (a sign of incompatibility) products which phase separate at room temperature.
In U.S. Pat. No. 4,234,714 a process for making prepolymers with hydroxy terminated diene polymers with aromatic isocyanates is described. Following the process of this reference, prepolymers based on hydroxy terminated liquid diene polymers and normally liquid relatively non-volatile aromatic polyisocyanates, such as trihydrocarbyl phosphate-modified methylene-bis-(phenylisocyanate) and polyarylene polyisocyanate, are made at a temperature above about 150.degree. C. for 1 to 4 hours. The resulting prepolymer is said to resist the tendency to form multiple phases or layers upon storage at room temperature. However, 4,4'-MDI prepolymers made by this process still has some hazy pearlescence, a sign of incompatibility.
A major disadvantage of the process of U.S. Pat. No. 4,234,714 is the extremely high process temperature required to make a phase stable prepolymer. The high temperature limits the production potential of the process. At high temperatures the polydiene polymers can homopolymerize, which increases the molecular weight of the polymer and the solution viscosity, and in extreme cases can gel the product. In addition, the high process temperature can cause the isocyanate to undergo dimerization, or form allophanates, which would also cause the solution viscosity to increase.
There is therefore a need for a phase-stable, liquid, MDI-based isocyanate functional polydiene prepolymer which can be prepared at much lower temperatures, desirably at 100.degree. C. or less, and which does not produce phase separation or pearlescence at ambient temperature storage.